fix(eval): sanitize example_id to prevent path traversal (fixes #78, #79)

[dex0shubham]

Problem

example_id is interpolated straight into output filenames via os.path.join in
eval/lib/pixel_query.py, with no sanitization. An id containing path separators
(e.g. ../../evil) makes the renderer write files outside the intended
output_dir — a path-traversal write. example_id comes from eval dataset records,
so the exposure is running the eval harness over an untrusted/crafted dataset.

Reported in #78 and #79.

What I changed

Added a small _safe_stem() helper (Path(x).name, which keeps only the final path
component — never a separator, never absolute) and applied it at all three affected
sites:

• PixelQueryRenderer.render → {example_id}_query.png (🔒 Path traversal vulnerability in output file path #78)
• PixelQueryRenderer.render_all → {eid}_query.png — same bug, not flagged in the
  issues but fixed here for completeness
• QueryImageTextRenderer.render → {example_id}_query_card.png (🔒 Path traversal vulnerability in QueryImageTextRenderer.render #79)

No public signature changes; legitimate ids (no separators) are completely unaffected.

Tests

Added tests/test_pixel_query_paths.py (runs under the existing pytest tests/ CI job):

• _safe_stem strips ../, absolute paths, nested and Windows-style separators.
• Constructed output paths for a battery of malicious ids resolve inside output_dir.
• Normal ids are preserved (abc123 → abc123_query.png) — no regression.
• A font-guarded integration test actually calls render("../evil", ...) and asserts the
  file lands inside output_dir and no file escapes (skips cleanly where no TTF is present).

All 14 tests pass; ruff check and ruff format --check are clean.

Closes #78. Closes #79.

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